Microfluidics for bacterial chemotaxis.

Microfluidics is revolutionizing the way we study the motile behavior of cells, by enabling observations at high spatial and temporal resolution in carefully controlled microenvironments. An important class of such behavior is bacterial chemotaxis, which plays a fundamental role in a broad range of processes, including disease pathogenesis, biofilm formation, bioremediation, and even carbon cycling in the ocean. In biophysical research, bacterial chemotaxis represents a powerful model system to understand how cells and organisms sense and respond to gradients. Using microfluidics to study chemotaxis of free-swimming bacteria presents experimental challenges that are distinct from those arising in chemotaxis studies of surface-adherent cells. Recently, these challenges have been met by the development of advanced microdevices, able to generate flow-free, steady gradients of arbitrary shape. Much attention to date has been focused on tool development. Yet, we are now at an exciting turning point where science begins to balance technology. Indeed, recent microfluidic studies provided new insights on both the mechanisms governing bacterial gradient sensing (e.g. tuning of response sensitivity, discrimination between conflicting gradients) and the large-scale consequences of chemotaxis (e.g. in the oceans). Here we outline the principles underlying recently proposed gradient generators for bacterial chemotaxis, illustrate the advantage of the microfluidic approach through selected examples, and identify a broader set of scientific questions that may now be addressed with this rapidly developing technology. The latest generation of microfluidic gradient generators, in particular, holds appeal for both biophysicists seeking to unravel the fundamental mechanisms of bacterial chemotaxis, and ecologists wishing to model chemotaxis in realistic environments. Time is ripe for a deeper integration between technology and biology in fully bringing to bear microfluidics on studies of this fascinating microbial behavior.